Google today launched a new browser tool that enables users to add and view comments directly on any Web page they view while cruising the Web.

Sidewiki, the new tool, comes as a new feature of Google Toolbar, the company's browser add-on for Internet Explorer and Mozilla Firefox.

Once installed and activated, a pane appears in the left-hand column of a user's Web browser. Within that area, users can view and contribute comments. Comments can be linked to one particular area of a page -- selectable by highlighting -- and also can be shared via Facebook, Twitter and e-mail, and on Google's Blogger service.

Site operators, once verified through Google Webmaster Tools, can also add their own posts to their Web page's Sidewiki, which will highlight the entries near the top of the tool's pane.

Today's launch marks the search giant as latest in a long line of companies that have brought out Web page commenting tools. None have grabbed a huge following, but Google (NASDAQ: GOOG) said it's worked on features it thinks users will find useful.

"In developing Sidewiki, we wanted to make sure that you'll see the most relevant entries first. We worked hard from the beginning to figure out which ones should appear on top and how to best order them," said a blog post written by Sundar Pichai, vice president of product management and Michal Cierniak, engineering lead for Google Sidewiki.

Instead of displaying the most recent entries first, Sidewiki rank entries using an algorithm that Google says promotes the most useful, high-quality entries in real-time. Criteria includes feedback from the user of Sidewiki as well as other users, previous entries made by the same author and many other "signals" Google says it's developed. Ranking technology and expertise is at the core of Google's search engine.

In addition to being released as a feature of Google Toolbar for Firefox and Internet Explorer, Google said it plans to make Sidewiki available for its own Google Chrome browser as well as others.

The company said it's been testing Sidewiki with several news organizations and "experts" before today's release.

Google also said it has released the first version of an API today that's designed to let anyone work freely with the content that's created in Sidewiki.